1. Field of the Invention
The present invention relates to a stretched resin film having good liquid absorbability with good seepage resistance, which, when stuck to objects with a water-base adhesive, dries within a short period of time and attains good adhesiveness. The invention also relates to a stretched resin film which is fabricated by coating the stretched resin film as above with a water-base coating layer usable in inkjet recording paper, thermal recording paper, thermal transfer recording paper, pressure-sensitive transfer recording paper and other various types of printing paper. The invention further relates to a stretched resin film which is fabricated by coating the stretched resin film as above with a water-base heat-seal resin coating layer usable in in-mold labels or various heat-sealable bags, and to products produced by the use of it. The invention still further relates to a label that is readily delabelable by washing with water, not requiring washing with a heated sodium hydroxide solution in a process of removing it from returnables, and to an object with the label stuck thereto.
2. Description of the Related Art
1. Heretofore, water-resistant film-type synthetic paper is used principally for adhesive labels with an organic solvent-containing adhesive applied thereto, and for mediums for offset printing, seal printing or sublimation or fusion-type thermal transfer recording that uses a solvent-type ink. These days, however, environmental protection is a matter of great concern in the art, and it is recommended to use ecological and safe water-base adhesives and water-base inks in place of organic solvent-containing adhesives and solvent-type inks. Accordingly, synthetic paper is needed which has good absorbability for such water-base adhesives or water-base inks and for water that serves as the solvent for these. Some types of synthetic paper have heretofore been proposed (JP-A-2001-181423, 2001-226507, 2001-164017, 2001-151918, 10-212367), some of which are used in glue labels or inkjet recording mediums.
Such conventional synthetic paper is characterized in that its water absorption is large, but on the other hand, it is problematic in that the rapid driability on the substrate surface thereof is unsatisfactory, concretely, the water absorption rate on the water-absorbing surface thereof is not high. Accordingly, conventional synthetic paper is unsatisfactory for use in glue labels that require rapid driability of water-base adhesives thereon. For example, when high-speed bottle labeling is tried with conventional synthetic paper, then satisfactory initial label adhesion could not be attained and labels may shift or peel away during transportation of bottles. Accordingly, there is a substantial problem in that the bottle-labeling speed is difficult to increase and the production cost could not be reduced. In addition, in a process of printing patterns or letters with a water-base ink, the printing speed could not be satisfactorily increased when conventional synthetic paper is used.
In consideration of these problems with the related art as above, an object of the present invention is to provide a stretched resin film which has good water absorbability and is excellent in rapid driability of a water-base adhesive and water-base ink applied thereto.
2. For recording mediums for inkjet recording paper, thermal recording paper, thermal transfer receiving paper, pressure-sensitive transfer recording paper, paper for outdoor exhibition posters and various labels that require water resistance, weather resistance and durability, heretofore proposed are those fabricated by coating a stretched resin film with a water-base coating agent in various coating lines, in which the stretched resin film includes synthetic paper fabricated in a process of film formation that comprises blending a polyolefin-based resin or a polyester-based resin with an inorganic fine powder or an organic filler followed by forming the resulting blend into a film and stretching it, and a transparent stretched film of essentially a polyester-based resin (see JP-A-10-188188, claim 1; JP-A-2001-181423, paragraphs [0079] to [0083]; JP-A-2001-246830, claim 1; JP-A-2001-253166, claim 1; JP-A-2002-46346, claim 1; JP-A-2002-46350, claim 1; JP-A-2002-46351, claim 1; JP-A-5-124335, claim 1; JP-A-5-1969806, claim 1; JP-A-5-169804, claim 1). However, many cases of these that are fabricated by coating the surface of such a stretched resin film with a water-base coating agent are problematic in that, since the water absorbability of the stretched resin film is insufficient, the coating agent is repelled and becomes uneven owing to the poor wettability of the stretched resin film with the water-base coating agent, and the adhesiveness between the coating layer and the stretched resin film is poor.
To solve these problems, the surface of the stretched resin film must be previously undercoated with a wettability improver or an anchor-coating agents. The additional undercoating step increases the production cost. When the wettability of the stretched resin film with an aqueous solution is not good or when the ability of the film to absorb an aqueous solution is poor, then the viscosity and the solid concentration of the water-base coating agent will have to be controlled.
Another object of the invention is to provide a stretched resin film coated with a water-base coating layer, of which the advantages are that, when the film is coated with the water-base coating layer, it is free from a problem of coating agent repellency and unevenness even though it is not undercoated with an anchor-coating agent heretofore needed in in-line and/or out-line coating of stretched resin films, and therefore the limitation on the water-base coating agent applicable to the stretched resin film for use in the invention is relieved and the adhesiveness of the coating layer to the film is thereby increased.
3. In-mold labels have heretofore been developed, which are used in the field that requires water resistance, chemical resistance and durability. For example, they include labels fabricated by coating one surface of a white film (synthetic paper) formed of a stretched or unstretched transparent polyolefin-based resin film or a stretched or unstretched, inorganic fine powder or organic filler-containing polyolefin-based resin film, with a heat-seal resin layer in a mode of coextrusion for film formation (JP-A-2-84319), labels fabricated by attaching or laminating a previously-embossed heat-seal resin film onto a substrate, and labels fabricated by coating a substrate with a heat-seal resin in various coating lines (JP-A-2-122914), and some of them have been commercialized.
Such conventional in-mold labels require embossing on the surface of the heat-resin layer, attaching or laminating a previously-embossed film onto a substrate, or forming a trapezoidal pattern in labeling, in order to remove the air trapped in a space between the container and the label in the in-mold process of producing labeled containers. When a pattern for air removal could not be formed on the surface of containers, then a method is proposed which comprises forming small holes in the label so as to remove air through them (JP-A-2-108516) However, any of these additional steps complicates the production process and could not evade various problems in that the producibility lowers, the number of failed products increases and the production cost increases.
In the conventional process where a heat-seal resin layer is formed through coextrusion in the step of producing in-mold labels, there occurs a problem in that the usable heat-seal resin is naturally limited depending on the heat resistance, the corrosion resistance and the viscosity behavior in melt of the resin.
In the other conventional process where a substrate is coated in various coating lines, when the surface of a previously-embossed stretched resin film is coated with a heat-seal resin (JP-A-5-249895), then there occurs a problem in that, since the solvent absorbability of the stretched resin film is poor, most of the coating agent gathers in the recesses of the embossed stretched resin film in the step of drying it, and, as a result, the embossed pattern is difficult to reproduce on the coated surface. When a film of no water absorbability is used, then a method may be employed which comprises applying a high-viscosity coating agent of a hot-melt resin or a solvent-type heat-seal resin to the film by the use of a gravure-coating machine to thereby make the film have a trapezoidal pattern of the embossing roll cells. Even in this case, however, such high-viscosity coating is indispensable and there still occurs a problem in that the production cost increases owing to the reduction in the producibility and another problem with the case is that the organic solvent used may cause firing and environmental pollution.
Still another object of the invention is to omit the necessity of the additional step of forming a specific pattern on the surface of a heat-seal coating layer or forming small through-holes in the layer in forming the layer on the surface of a stretched resin film. Still another object of the invention is to provide a stretched resin film of good producibility and good in-mold labelability, of which the advantages are that the latitude in selecting the material of constituting the heat-seal resin coating layer for it can be broadened and the limitation on the coating and processing equipment for it is relieved. Still another object of the invention is to provide an in-mold label having excellent functions and to provide in-mold containers with the label stuck thereto.
4. For various drink containers, these days used are one-way containers that are recycled from a melt of recovered material or cullet, and returnable containers that are collected, washed and reused, after used. Especially for recycle society in consideration of environmental harmony, it is believed that use of returnable containers of low environmental load, which require small energy consumption and save natural resources and wastes, may further increase in future rather than one-way containers.
Beer bottles used in large quantities as such returnable containers are generally labeled for the purpose of improving their design decoration. One general structure of conventional returnable bottle labels comprises an aluminium deposition layer formed on one surface of natural paper for improving the water resistance and the design decoration thereof, and a water-base adhesive applied on the other surface thereof, and the label of the type is stuck to a bottle via the water-base adhesive thereof. In the step of reusing the returnable bottles, the recovered bottles are washed with water for pretreatment and then washed with a sodium hydroxide solution heated at 70° C. or so for dissolving the aluminium deposition layer. This is for completely removing the label from the bottles (delabeling). However, the delabeling process has various problems in that the sodium hydroxide solution is harmful to human bodies, dissolution of the aluminium deposition layer and washing inside the bottles produce wastes, repeated use of the sodium hydroxide solution for about 1 year produces a problem in sanitation, and the glass bottles themselves are corroded. Accordingly, it is desired to develop a label that may be readily delabeled merely by washing with water for pretreatment, not requiring use of heated sodium hydroxide solution.
Conventional labels are formed of natural paper, and therefore require the aluminium deposition layer for making them resistant to water and for improving their design decoration. In that situation, it may be considered to use film-type synthetic paper of good water resistance in order to omit the aluminium deposition layer on it. In general, however, film-type synthetic paper has poor water absorbability and is therefore problematic in that a water-base adhesive is difficult to use for it.
On the other hand, there are proposed synthetic paper suitable for water-base inkjet recording paper of good water absorbability (JP-A-2001-226507, 2002-96422, 2001-164017, 2001-151918, 2001-181423), and synthetic paper to which water-base adhesives are applicable (JP-A-10-212367). However, even when such synthetic paper is used, it is still difficult to produce labels that are readily delabelable by washing with water alone.
In consideration of the problems with the related art as above, still another object of the invention is to provide a label that is readily delabelable by washing with water alone, not requiring washing with a heated sodium hydroxide solution in a process of removing it from returnables.